This invention relates generally to a method and system for producing a PPI display and more particularly to a technique and recording system for recording, on a two-dimensional PPI display device data obtained from borehole scanning operations.
It is frequently necessary to measure the diameter, or cross sectional area, of a borehole which may extend to great depth. In addition to determining the actual configuration of the borehole, it may be desirable to determine anomalies which may exist at different depths in the borehole. For example, the anomalies may be a fault or crack in the borehole, or in a well casing. Perforations of the well casing may need to be inspected. In other situations, it may be desirable to determine the existence of faults, or other characteristics, in the adjacent formations spaced from the borehole.
A borehole logging system useful for making such determinations is described in U.S. Pat. No. 3,668,619 to Dennis. In such system, a borehole logging tool includes an acoustic transmitting means that is rotated and periodically operated to transmit acoustic energy pulses toward the borehole wall. The transducing means detects reflected energy for the production of reflection signals. During logging operations, an orientation signal is produced each time the borehole logging tool is rotated past a predetermined geographic orientation, such as magnetic north. These orientation signals are employed to produce two sine and cosine functions. The orientation pulses trigger a function generator which produces the sine function. The cosine function is then produced from the sine function.
The acoustic reflection signals and the sine and cosine functions are applied to a PPI recording system comprising a display device, two electron beam deflection means, and an electron beam intensity modulating means. The two orientation functions sine and cosine are applied to the electron beam deflection plates on the PPI display so as to cyclically reproduce a circular rotating beam sweep. The reflection signals are applied to the video input of the PPI display to intensify the electron beam for the display of a trace pattern reflecting information representative of variations in the distance between the transducing means and the wall of the borehole or other subsurface anomalies within or surrounding the borehole. A somewhat similar system is described in U.S. Pat. No. 3,369,626 to Zemanek.